1. Field of the Invention
The present invention relates to analog-to-digital (AD) converters (abbreviated “ADC”) and semiconductor devices, such as imagers, provided with the ADCs. More particularly, the present invention relates to the mechanism of so-called single-slope-integration or ramp-compare AD conversion for comparing a reference signal that changes with a predetermined slope with an analog signal to be processed and counting the comparison time.
2. Description of the Related Art
In electronic apparatuses, such as cameras, cellular phones, and imaging modules, various ADCs for converting an analog signal into a digital signal are used.
For example, physical-quantity-distribution sensing semiconductor devices including a line or a matrix of a plurality of unit elements (for example, pixels) responsive to electromagnetic waves, such as light or radiation, input from an external source are used in various fields.
In the video equipment field, for example, charge-coupled device (CCD), metal oxide semiconductor (MOS), or complementary metal-oxide semiconductor (CMOS) solid-state imagers for sensing light (an example of electromagnetic waves) as the physical quantity are used. Such imagers read the physical quantity distribution obtained by converting light into an electric signal using the unit elements (pixels in the solid-state imagers) as the electric signal. The word “solid-state” means that the imagers are made of semiconductor.
The solid-state imagers include amplifying solid-state imagers including active pixel sensor (abbreviated “APS” or called “gain cell”) pixels provided with drive transistors for amplification disposed in pixel signal generators for generating pixel signals in accordance with signal charges generated by charge generators. For example, many of CMOS solid-state imagers have such a configuration.
In this type of APS imager, to read pixel signals to an external source, address control is performed on a pixel portion including an array of a plurality of unit pixels so that the signals are arbitrarily and selectively read from the individual unit pixels. That is, an APS imager is an example of an address-control solid-state imager.
For example, in an APS imager, which is one type of an X-Y address solid-state imager having unit pixels disposed in a matrix, MOS active elements (MOS transistors) are included in pixels so that the pixels themselves have an amplifying function. That is, signal charges (photoelectrons) stored in photodiodes, which are photoelectric conversion elements, are amplified by the active elements and read as image information.
In this type of X-Y address solid-stage imager, for example, many pixel transistors are disposed in a two-dimensional matrix to form a pixel portion, the accumulation of signal charges in accordance with incident light in each line (row) or each pixel is started, and current or voltage signals based on the accumulated signal charges are sequentially read from the individual pixels by addressing.
In MOS (including CMOS) solid-state imagers, an address control method for accessing the pixels in one row at one time and reading out the pixel signals from the pixel portion in units of rows is mostly used. The analog pixel signals read from the pixel portion are converted into digital signals using ADCs, if necessary, and then output to an external source (for example, see Japanese Unexamined Patent Application Publication Nos. 2000-152082 and 2002-232291 and U.S. Pat. Nos. 5,877,715, 5,920,274, and 6,344,877).
As has been described in these patent documents, there are various AD conversion methods in terms of the circuit scale, the processing speed, the resolution, etc. Among these methods, there is a so-called single-slope-integrating or ramp-compare AD conversion method for comparing an analog unit signal with an AD-conversion reference signal that changes monotonously, performing counting in parallel with comparison, and obtaining a digital signal of the unit signal on the basis of the count value at the time comparison is completed.